Pope Benedict urged lapsed and lukewarm Roman Catholics on Thursday to rediscover their faith but acknowledged there are "bad fish" in the Church itself.

The pope made his comments at two large events before thousands of people in St. Peter's Square on the 50th anniversary of the opening of the Second Vatican Council, a far-reaching event in the Church's 2,000-year history.

"Recent decades have seen the advance of a spiritual desertification," he said in his sermon of a morning Mass, opening a worldwide "Year of Faith".